1. Field of the Invention
The present invention relates to a fourth harmonic generating system using an optical double side-band suppressed carrier modulator and the like.
2. Description of the Related Art
As an optical modulator, an optical double side-band suppressed carrier modulator (DSB-SC modulator) is known. Specifically, FIG. 37 of Japanese Patent Application Laid-Open No. 2004-252386, for example, discloses a DSB-SC modulator having a Mach-Zehnder (referred to below as MZ) which has phase modulators on its both arms and a fixed phase shifter on one of its arms. A DSB-SC modulator, ideally, outputs two side-band (double side-band) signals, suppressing carrier signal components. In other words, USB (f0+fm) and LSB (f0−fm) can be obtained by suppressing carrier (f0) by means of the DSB-SC modulator.
Accordingly, it can be assumed that a signal of a frequency (f0+2fm) and a signal of a frequency (f0−2fm) can be obtained by shifting a frequency of the DSB-SC modulator from (fm) to (2fm). However, since a frequency of a modulating signal is constrained by a modulation signal source of a system, frequency difference between an USB signal and an LSB signal is limited.
It is conceivable that two DSB-SC modulators are connected in tandem (connected in series) in order to generate a signal of a frequency (f0+2fm) and a signal of a frequency (f0−2fm) by the DSB-SC modulator. However, if an output signal, which is modulated by the first DSB-SC modulator, is further modulated by the second DSB-SC modulator, the side-band frequencies of these signals are equal at carrier frequency (f0). Therefore, in a system in which two DSB-SC modulators are connected in tandem, signals of frequencies (f0+2fm), (f0), and (f0−2fm) exist. This means that the signal of frequency (f0) remains with some signal strength.
The present invention is made to provide an optical modulating system for generating a signal of a frequency (f0+2fm) and a signal of a frequency (f0−2fm) by suppressing a signal of a frequency (f0) by means of a DSB-SC modulator.
The present invention is made to provide an optical modulating system for generating signals whose frequencies (f0±2fm) are separated by four times a modulating frequency (fm) by means of a DSB-SC modulator.
The present invention is made to provide a method for generating an optical signal of a frequency (f0+2fm) and an optical signal of a frequency (f0−2fm) by suppressing an optical signal of a frequency (f0) as much as possible by means of a DSB-SC modulator.